Freeze Drying Is Not a Kill Step — and Four Other Things Worth Teaching

Freeze drying does not kill bacteria. It applies no heat treatment, so any organism present in the food goes into the machine, survives in a dormant state, and can resume growing the moment moisture is added back. Every food you freeze dry needs a kill step somewhere — either before it goes in, or after it is rehydrated.

That single fact is the thing most new owners get wrong, and it is the reason we put together a six-page reference for Extension volunteers, food-preservation instructors and fair superintendents who are being asked to teach or judge freeze drying for the first time. It is free, it carries no sales pitch, and it is written to be copied, excerpted and handed out. Download it here.

A word on where the authority sits. We build freeze dryers; we are not a food-safety authority. Every safety statement in the reference is attributed to the Cooperative Extension or National Center for Home Food Preservation source it came from, with links, so you can verify it and cite it directly in your own materials. Where the published sources disagree, we say so rather than quietly picking one. The sections on machine behaviour are drawn from our own testing and are labelled as such.

There is no tested-process standard yet

This is the part that surprises people coming from canning. Home freeze drying has no USDA-validated process. The National Center for Home Food Preservation has not published research-based freeze-drying recipes, and Extension notes that validation is genuinely difficult because home units differ from one another in trap temperature, vacuum and shelf control.

Utah State, Minnesota, Arizona and Penn State have all published guidance in the last two years, and that guidance is where the current consensus lives. But there is no schedule to judge an entry against the way you would judge a jar of green beans against a tested pressure-canning schedule.

The workable alternative is to judge the practice: whether the food was an appropriate choice for the method, whether the required kill step happened and is documented, whether the product is genuinely dry, and whether it is packaged in a way that will keep it that way.

Where the two main sources disagree

Any committee writing rules has to settle this one explicitly, because the two most-cited Extension publications do not agree on raw protein.

• Utah State Extension treats raw meat, poultry and seafood as unsuitable: cook to 160 °F, 165 °F and 145 °F respectively before freeze drying.
• University of Minnesota Extension permits them to be dried raw or cooked, on the condition that anything needing cooking is cooked to the safe minimum internal temperature after rehydration.

Both agree there must be a kill step. They disagree about which end of the process it sits at.

For a fair class we suggest requiring the kill step before drying, documented on the entry card. A fair exhibit is handled, displayed at room temperature for days, sometimes opened and sometimes tasted, and nobody can guarantee a later cook step will happen. Taking the stricter of the two published positions costs an exhibitor nothing and removes the whole question from the judging table.

For your own kitchen, either published position is defensible. Just pick one deliberately instead of drifting.

The foods that are simply a no

The full food-by-food table in the reference runs to seventeen rows. These are the ones worth memorising, because no setting change rescues them:

• Raw milk and unpasteurized dairy. Listeria, Salmonella and E. coli risk. Freeze drying does not remove it.
• Raw flour and dough. Can carry E. coli and Salmonella, and no kill step is applied.
• Raw egg. Use pasteurized liquid egg or fully cooked scrambled egg instead.
• Oils, mayonnaise and dressings. Fat does not sublimate. The product will not dry, and it goes rancid in storage.
• Sausage in casings, very fatty cuts, alcohol. Known to dry poorly or not at all.

Wild game gets its own rule: for parasites, cut into pieces 6 in. thick or less and freeze at 0 °F or below for 30 days before processing, then follow the rule for meat.

Vegetables need blanching exactly as they would for freezing — raw broccoli shrivels badly. Grapes dried raw turn into raisins. If you want the microbiology behind what survives the process and why, we went into it separately in bacterial resilience in freeze drying.

How to actually tell it is dry

There is no colour change and no timer to rely on. A machine reporting "complete" is telling you its programme finished, not that the food is dry. Three checks, used together:

1. Break test. The piece should be crisp, dry or brittle. Any pliability, softness or sponginess means moisture remains.
2. Cut-and-look. Cut a piece in half and look at the centre for a darker core. One dark centre means the whole batch goes back in. Test a piece from every tray — units dry unevenly top to bottom.
3. Cold spot. A piece that feels cool to the touch is still subliming. Cool means wet.

The most reliable method, and the one worth teaching, is the weight test: weigh the loaded trays before the cycle and again after. When the weight stops falling between checks, the food is dry. It also tells the preserver how much water to add back at rehydration.

One machine note from our own testing: if a batch tests wet, add more final dry time rather than restarting the whole cycle. The food is already frozen and under vacuum, and re-running from the start wastes most of a day.

The 25-year number is wrong, and it is worth saying out loud

The familiar 25-year figure is a commercially processed and packaged figure. Extension guidance puts well-packaged home-processed product at 10–15 years, and recommends not relying on home product beyond 5–10 years. Vitamin C in particular can fall substantially within one to three years regardless of how good the seal is.

Most people arrive at a class having heard 25 years. Correcting it early changes how they think about packaging, which is the part that actually determines whether their food survives.

Packaging is a third of the outcome

Freeze-dried food is hygroscopic and begins reabsorbing room moisture the moment the chamber opens. Minutes matter on a humid day.

• Use a real moisture barrier: mylar at 5–7 mil, with no clear window. Windowed bags admit measurably more air and moisture, and many vacuum-sealer bags are not true barriers.
• Seal promptly with an impulse sealer, with an oxygen absorber sized to the container.
• Store cool, dark and dry, and label with the food, whether it was raw or cooked, and the date.

We covered the mechanics in more depth in packaging freeze-dried food for long-term storage. A bag that has gone soft weeks later is not a cosmetic problem — water activity has risen and dormant organisms can become active again.

If you are setting up a fair class, you are early

We looked for existing precedent before writing this. As of the 2026 fair books, freeze-dried classes are still rare: the Maryland State Fair preservation section runs dried-food classes K44–K57 with no freeze-dried class, and University of Nebraska county fair books likewise list dried fruit, leather, vegetables and herbs only.

The practical approach is to borrow the dried-foods class template, which exhibitors and judges already understand, and add the one thing freeze drying has that dehydrating does not: recorded machine settings.

The reference includes eight suggested classes (fruit, vegetables, herbs, cooked meat or poultry, pasteurized dairy, complete meal, candy, and a rehydration demonstration), a 100-point score sheet, a list of automatic disqualifiers, and a one-page process card to require with every entry.

That process card is the part we would argue hardest for. It records the four settings common to home freeze dryers — shelf chill, target vacuum, maximum shelf temperature and final dry time — alongside pre-treatment, dryness test used and packaging. An exhibitor who cannot report those settings did not control the process, and a judge who has them can explain exactly why an entry scored as it did. It also turns the class into a teaching instrument, because the cards accumulate into a local record of what settings actually worked.

Download the reference

Six pages, compiled October 2026, free to copy and hand out: Freeze Drying: Safety, Process and Judging Reference (PDF).

No email required and nothing to sign up for. If you are an Extension educator or a fair superintendent and something in it conflicts with your state's guidance, we would rather hear about it than have it circulate uncorrected — tell us and we will fix the next revision.

It covers, in order: what freeze drying does and does not do; the absence of a tested-process standard; the raw-protein disagreement; the food-by-food table; preparation; determining dryness; packaging and storage; rehydration; common failures and their causes; setting up a fair class; the draft score sheet; and a sources page with direct links to the Utah State, Minnesota, Arizona and NCHFP publications it draws on.

If you are still choosing a machine for a teaching programme or a school, our buyer's guide and the full specifications page cover the capacity and control questions, and the vials guide covers laboratory use.

Sources

All of the following are free and citable in class handouts.

• Utah State University Extension, Can I Freeze Dry That? A Practical Guide to Safe and Effective Freeze Drying — the most complete food-by-food safety guide published.
• Utah State University Extension, Buying a Home Freeze-Dryer: What You Need to Know Before You Go — neutral purchase-decision guidance.
• University of Minnesota Extension, Freeze drying — dryness testing, wild game, packaging, and the clearest statement of where tested processes stand.
• University of Arizona Cooperative Extension, Freeze Drying 101 (az2097a, Jan 2025) — a peer-reviewed four-part handout series written for teaching.
• National Center for Home Food Preservation, Expanding Your Home Food Preservation Toolkit with Freeze-drying — recorded webinar, Feb 2026, Penn State.

Safety guidance above is quoted or paraphrased from the sources named and is not Stay Fresh's own. Please verify against the current edition of each publication before putting it into fair rules or class materials.